The present invention relates generally to optical inspection systems for detecting defects on a sample. More specifically, it relates to mechanisms for filtering noise from the detection of defects within such optical systems.
Many instruments currently available for detecting small particles on wafers, reticles, photo masks, flat panels and other specimens use darkfield imaging. Under dark field imaging, flat, specular areas scatter very little signal back at the detector, resulting in a dark image, hence the term dark field. Surface features and objects that protrude above the surface scatter more light back to the detector. In darkfield imaging, the image is normally dark except areas where particles or circuit features exist. A darkfield particle detection system can be built based on the assumption that particles scatter light more than circuit features.
In darkfield type optical inspection systems, an intense light beam in the visible wavelength range is directed towards a sample. Light scattered from the sample in response to such incident beam is then collected by a detector. The detector generates an image of the sample from the scattered light. Since defects, such as particles or voids, cause the incident light to scatter, scattered light may indicate the presence of such a defect. However, other features of the sample that are not defects may cause the incident beam to scatter resulting in the detection of xe2x80x9cfalsexe2x80x9d or xe2x80x9cnuisancexe2x80x9d defects. For example, repeating patterns on the sample, which are typically present on a semiconductor device, cause incident light to scatter so that sharp bright spots are imaged on the detector. These sharp bright spots may obscure actual defects. Additionally, line features at specific angles on a sample may result in scattering in broad regions of the image at specific angles, e.g., 45xc2x0 and 90xc2x0, which also obscures detection of xe2x80x9creal defects.xe2x80x9d
There are instruments that address some aspects of the xe2x80x9cnuisance defectxe2x80x9d problems associated with darkfield. One method in use today to enhance the detection of particles is spatial filtering. Under plane wave illumination, the intensity distribution at the back focal plane of a lens is proportional to the Fourier transform of the object. Further, for a repeating pattern, the Fourier transform consists of an array of light dots. By placing a filter in the back focal plane of the lens which blocks out the repeating light dots, the repeating circuit pattern can be filtered out and leave only non-repeating signals from particles and other defects under certain ideal conditions.
Although conventional Liquid Crystal type spatial filters work well within inspection systems that operate in the visible light range, they fail to effectively inhibit light in the ultraviolet (UV) region from nuisance sources from reaching the detectors. A UV light source may be used for any number of reasons, e.g., to effectively detect smaller sized defects. However, a conventional spatial filter""s extinction capability is greatly reduced when going from a visible to an UV light source. Additionally, a conventional spatial filter fails to effectively transmit scattered light in the UV region. In a specific example, a conventional PSF has an extinction value of 200:1 and a transmission value of 20 percent at the UV wavelength of 364 nm. It should be noted that the extinction ratios described herein are measured with a detector that is at a distance of 10 inches from the filter and has an aperture of 1 centimeter. Finally, both the transmission and extinction performance of conventional spatial filters degrade over time under UV light exposure.
Accordingly, there is a need for an improved liquid crystal type programmable spatial filter for use in a darkfield optical inspection system that has improved extinction and transmission performance in the UV region.
Accordingly, mechanisms are provided for selectively filtering spatial portions of light emanating from a sample under inspection within an optical system. In one embodiment, a programmable spatial filter (PSF) is constructed from materials that are compatible with light in a portion of the UV wavelength range. In a specific implementation, the PSF is constructed from a UV compatible material, such as a polymer stabilized liquid crystal material. In a further aspect, the PSF also includes a pair of plates that are formed from a UV grade glass. The PSF may also include a relatively thin first and second ITO layer that results in a sheet resistance between about 100 and about 300 xcexa9 per square. The PSF provides selective filtering in two directions. In other words, the PSF provides two dimensional filtering.
In one embodiment, an optical inspection system for detecting anomalies on a sample is disclosed. The inspection system includes a light source for directing an incident light beam onto a sample and a programmable spatial filter (PSF) arranged in a path of light emanating from the sample in response to the incident light beam. The PSF is configurable to inhibit or transmit one or more selected portions of the emanating light, and the selected portions of the emanating light may be selected from a plurality of emanating light portions arranged along a first direction, as well as in a second direction. The first direction differs from the second direction. The inspection system further includes a detector arranged within the path of the emanating light so that the second portion of the emanating light that is transmitted by the PSF impinges on the detector to thereby form an image of at least a portion of the sample and an analyzer for receiving the image and determining whether there are any defects present on the sample portion by analyzing the received image.
In one implementation, the first direction is perpendicular to the second direction. In a further aspect, the sub-regions are arranged to facilitate inhibition of portions of the emanating light at a particular angle resulting from patterns on the sample. For example, the angle is 45 or 90 degrees from a plane of incidence. In a specific implementation, the PSF has a pair of plates formed from a material that substantially transmits ultraviolet light and are sized to cover an aperture of the emanating light, the plates being arranged parallel to each other, a first indium tin oxide (ITO) layer deposed on a first one of the plates, and a second ITO layer deposed on a second one of the plates. The first and second ITO layer are positioned between the plates. The first and/or the second ITO layer is divided into portions along both the first and second directions. The liquid crystal layer is arranged between the first ITO layer and the second ITO layer portions.
In a further embodiment, the first ITO layer is divided into portions and the second layer is not. The system further includes a controller configured for selectively apply a voltage potential difference between at least a one of the first ITO layer portions and the second ITO layer so that an adjacent portion of the liquid crystal layer allows transmission of a first portion of the emanating light while another portions of the liquid crystal layer on which a voltage potential difference is not applied inhibits a second portion of the emanating light through the PSF. In one aspect, the voltage potential difference is greater than a predetermined threshold, and the adjacent portion of the liquid crystal layer allows transmission when a voltage potential difference is applied that is greater than the predetermined threshold. In an alternative embodiment, the voltage potential difference is less than a predetermined threshold, and the adjacent portion of the liquid crystal layer allows transmission when a voltage potential difference is applied that is less than the predetermined threshold.
In a specific embodiment, the first ITO layer is divided into a plurality of portions that are arranged in a plurality of rows and the second ITO layer is divided into a plurality of columns. The system includes a controller configured to apply a first voltage potential difference between a selected row and a selected column so that a portion of the liquid crystal layer that is adjacent to an intersection of the selected row and column changes from being configured to inhibit to being configured to allow a corresponding portion of the emanating light. In a further aspect, the controller is further configured to apply a second voltage potential difference between the selected row and the selected column that previously had a first voltage applied there between so that the portion of the liquid crystal layer that is adjacent to the intersection of the selected row and column remains configured to allow the corresponding portion of the emanating light. The second potential difference is lower than the first potential difference. In yet a further aspect, the controller is further configured to apply a third voltage potential difference between the selected row and the selected column that previously had a second voltage applied there between so that the portion of the liquid crystal layer that is adjacent to the intersection of the selected row and column changes from being configured to allow to being configured to inhibit the corresponding portion of the emanating light. The third potential difference is lower than the second potential difference. In a further implementation, the controller is further configured to apply a fourth voltage potential difference between the selected row and the selected column that previously had a third voltage applied there between so that the portion of the liquid crystal layer that is adjacent to the intersection of the selected row and column remains configured to inhibit the corresponding portion of the emanating light. The fourth potential difference is higher than the third potential difference. In a preferred embodiment, the liquid crystal layer is a bi-stable material.
In another embodiment, the invention pertains to a method of detecting anomalies on a sample. An incident light beam is directed onto a sample. A first spatial portion of a emanating light beam, which results from the incident light beam hitting the sample, is selectively transmitted to a detector positioned within an image plane of such emanating light beam. The first spatial portion is transmitted by applying a first voltage potential difference between a first row and a first column of a programmable spatial filter (PSF) having a plurality of addressable rows and columns. A second spatial portion of the emanating light beam is inhibited through the PSF to the detector.
In a further aspect, the transmission of the first spatial portion of the emanating light is maintained by applying a second voltage potential difference between the first row and the first column. The second voltage potential difference is lower than the first voltage potential difference. In a further aspect, the first spatial portion of the emanating light is inhibited through the PSF from reaching the detector by applying a third voltage potential difference to the first row and the first column. The third voltage potential difference is lower than the second voltage potential difference. In another aspect, the inhibition of the first spatial portion of the emanating light is maintained by applying the second voltage potential difference between the first row and the first column. In yet another embodiment, the second spatial portion of the emanating light beam is inhibited from reaching the PSF the detector is accomplished by applying a second voltage potential to a second row and a second column, where the second voltage being lower than the first voltage.